Using Models to Understand the Lunar Interior

Credit: NASA/Colorado School of Mines/MIT/JPL/GSFC.

While there is general consensus that the Moon is the product of a large collision between Earth and a Mars-sized planet and subsequent widespread melting and crystallization, details of lunar formation and evolution remain the focus of intense study. In particular, understanding the internal structure of the Moon, such as its composition and density, has been limited to experiments done in laboratories on Earth. While extremely valuable, experiments are constrained by scientists’ ability to replicate the conditions of the lunar interior. For example, understanding the formation of a lunar magma ocean, which cooled and crystallized tens to hundreds of millions of years post-collision and produced the diversity of rock types observed on the Moon today, is limited to expensive, time-consuming experiments that only provide information on specific pressure-temperature-composition (P-T-X) conditions.

An alternative approach is thermodynamic modeling, which uses intrinsic material properties that are independent of a mineral’s mass, such as density and melting temperature, to predict mineral stability over a continuous range of P-T-X conditions for a given rock composition. Thermodynamic models are advantageous in that they can simulate a wide range of conditions in a relatively short time, giving scientists the freedom to test many hypotheses rapidly. However, models are also dependent on the quality of the datasets compiled of the intrinsic properties of minerals and have historically been developed to address terrestrial, not lunar, conditions. In a recent study led by Tim Johnson at Curtin University in Perth, Australia, scientists tested the application of thermodynamic models in non-terrestrial environments and explored various crystallization hypotheses for the lunar magma ocean.

The study predicted the density and composition of the lunar crust and mantle and found that results differed depending on the rock type (i.e., lunar basalt, lunar upper mantle) considered. Thus, model results support several current crystallization and melting hypotheses for the Moon. Furthermore, the study compared model predictions with published experimental results for the same conditions and compositions and concluded that thermodynamic models generally agree with previous experimental data. This result is important because it implies that in future studies, thermodynamic models can be used to quantitatively investigate melting and crystallization processes, which are important in forming and stabilizing crust, on other rocky bodies in addition to the Moon. Thus, thermodynamic models have the potential to help scientists better understand how a planet’s crust formed and ultimately evolved to what we observe today. READ MORE